1. Field of the Invention
The present invention relates to a stamper used in a process of producing an optical recording medium (hereinafter also referred to as an “optical disk”), an optical recording medium, and methods of producing the same.
2. Description of the Related Art
In recent years, along with the advances made in technology for recording moving pictures, still images, and other video data in a digital format, compact disk (CD), digital versatile disk (DVD), and other optical disk apparatuses have been spotlighted as large capacity recording apparatuses. Research is underway for further increasing their capacity.
FIG. 1 is a cross-sectional view of principal parts of an optical disk of the related art.
One surface of a light transmitting disk substrate 112 made of for example polycarbonate is provided with projecting regions 112a and recessed regions 112b defining track regions.
That surface has further formed on it an optical recording multilayer film 113 comprised for example of a dielectric film, a phase change film or other recording film, another dielectric film, a reflection film, etc. stacked in that order. The configuration and the number of layers of the optical recording multilayer film 113 differ in accordance with the type of the recording material and the design.
Furthermore, a protective film 114 is formed on the optical recording multilayer film 113.
In the above optical disk, when recording or reproducing information, a laser light is irradiated for example from the disk substrate 112 side to the optical recording multilayer film 113.
When reproducing from the optical disk, a light receiving element receives the return light reflected on the optical recording multilayer film 13, and a signal processing circuit generates a predetermined signal to give a reproduction signal.
In the above optical disk, the optical recording multilayer film 113 is also provided with relief shapes in accordance with the projecting regions 112a and recessed regions 112b provided on one surface of the disk substrate 112. The projecting regions 112a and recessed regions 112b define the track regions.
The projecting regions 112a are referred as “lands”, which are projecting to the protective film 114 side when viewed from the disk substrate 112 side, while the recessed regions 112b are referred as “grooves”. It is possible to record information both at the lands and the grooves or use only one of the lands and grooves as recording regions.
Further, by forming pits having lengths corresponding to the recorded data as the relief shapes of the disk substrate 112 and by using an aluminum film or other reflection film as an optical recording film instead of the optical recording multilayer film 113, it is possible to obtain a read only (ROM) type optical disk.
The capacity of optical disk apparatuses using such optical disks can be increased by shortening the wavelength of the laser light used for recording and reproduction of the optical disk and by increasing the numerical aperture (NA) of the object lens.
Further, the increase in the numerical aperture (NA) of the object lens leads to less tolerance to disk tilt in the optical disk apparatus, so to bring the coma within the allowable range, it is necessary to make the light transmitting layer thin.
For example, in a CD, the wavelength of the laser light is in the 780 nm band, the numerical aperture of the object lens is 0.45, and the thickness of the light transmitting disk substrate is 1.2 mm. On the other hand, in a DVD, the wavelength of the laser light is in the 650 nm band, the numerical aperture of the object lens is 0.6, and the thickness of a light transmitting disk substrate is 0.6 mm. For example, two disk substrates are bonded together for use as a substrate having a thickness of 1.2 mm.
As a next generation optical disk apparatus able to handle further increases in recording capacity, there has been proposed an optical disk apparatus shortening the wavelength of the laser light to the blue to bluish violet region (for example, 400 nm), increasing the numerical aperture of the object lens to 0.8 or more (for example, 0.85), and, corresponding to this, using an optical recording medium having a thickness of the light transmitting layer reduced to about 0.1 mm. Since the above light transmitting layer having a thickness of 0.1 mm has insufficient rigidity, a disk substrate having a thickness of about 1.1 mm is used.
A method of producing the above optical disk will be explained next.
FIG. 2A is a schematic view of the configuration of a cutting apparatus (exposure apparatus) used in the above method of production, while FIG. 2B is a perspective view of principal portions.
As a light source, for example a Kr gas (ion) laser GL having an oscillation wavelength of 351 nm is provided. On its light axis, an electro-optic modulator EOM, a polarized beam splitter PBS1, a mirror M1, a lens L1, an acousto-optic modulator AOM, a lens L2, a mirror M2, a lens L3, a lens L4, a dichroic mirror DCM, a mirror M3, and an object lens OL are arranged as optical elements at predetermined positions.
The optical elements of the mirror M2 and before are fixed on a fixed table, while the optical elements of the lens L3 on are mounted on a movable table MT.
Further, a resist disk RD′ obtained by forming a resist film R on a glass substrate G to be exposed by the cutting apparatus is chucked on an air spindle capable of being driven to rotate with a high rotational accuracy by a spindle motor.
A laser light LT1 emitted from the gas laser GL passes through the electro-optic modulator EOM and the polarized beam splitter PBS1 and is reflected at the mirror M1 to be bent in direction of advance. At this time, part of the laser light LT1 passes through the mirror M1 and strikes a photodiode PD1.
The laser light LT1 reflected at the mirror M1 in the above way is focused by the lens L1, passes through the acousto-optic modulator AOM and is returned to parallel light by the lens L2, and is reflected at the mirror M2 and bent in direction of advance. At this time, part of the laser light LT1 passes through the mirror M2 and strikes the photodiode PD2.
Depending on the intensity of the light received by the photodiode PD1 and photodiode PD2, automatic power control (APC) is performed to provide feedback to the electro-optic modulator EOM, and obtain a constant output.
Further, the acousto-optic modulator AOM modulates the laser light LT1 in accordance with need.
The laser light LT1 reflected at the mirror M2 is expanded in beam diameter by a beam expander comprising the lenses L3 and L4, passes through the dichroic mirror DCM, is reflected at the mirror M3, and is focused to a spot having a diameter of for example 0.3 μm on the resist film of the resist disk RD′ by the object lens to form an exposure area EX.
The above cutting apparatus can drive the resist disk RD′ to rotate in the spindle direction SD by a not shown spindle motor, feed the movable table MT in the radial direction of the resist disk RD′ in predetermined increments, and while doing this irradiate the above laser light LT1 to the resist film of the resist disk RD′ so as to expose a pattern on the resist disk RD′.
For example, in the process of producing a phase change type optical disk or other rewritable type optical disk, it is possible to make the exposure use laser light scan and expose the resist in a spiral along the pattern of the tracks (lands/grooves) defining the recording regions.
Further, in the process of producing a read only (ROM) type optical disk, it is possible to make the exposure use laser light scan and expose the resist in a spiral while turning it on and off in accordance with the recorded data (information pits).
An auto-focus (A/F) mechanism is provided to keep the focal point of the object lens in register with the resist disk RD′ at all times.
For example, an skew ray method using a laser diode LD having an oscillation wavelength of 680 nm as the A/F optical system is used. A laser light LT2 of 680 nm emitted from the laser diode LD is reflected on the light splitting surface of the polarized beam splitter PBS2, passes through a quarter wave plate QWP to be reflected at the dichroic mirror DCM, and strikes the resist disk RD′ together with the laser light LT1.
At this time, the laser light LT2 is focused by the object lens OL on the focal plane of the laser light. LT1.
The laser light LT2 is reflected at the resist disk RD′, passes through the object lens, and returns. The resultant spot is projected on a position sensing diode PSD.
At this time, by making the laser light LT2 strike substantially parallel to the light axis of the object lens from a position somewhat off from the light axis of the object lens and focus at a position somewhat off from the focal point of the laser light LT1 on the resist disk surface, displacement from the focal plane on the light axis of the resist disk surface is detected as displacement of the laser light LT2 on the resist disk. By the optical lever principle, it is detected magnified about 100-fold on the position sensing diode PSD.
In this way, the position of a spot on the position sensing diode PSD is detected, the displacement from the position of the spot of the return light of the laser light LT2 at the time when the resist disk surface is in register with the focal position is fed back as an amount of focal error to an actuator for vertical movement of the object lens to drive the actuator, and A/F servo control is performed to keep the laser light LT1 focused on the resist disk at all times.
Further, the A/F optical system uses the polarized beam splitter PBS2 and the quarter-wave plate QWP as polarization elements for effectively separating an outbound path and return path of the laser light LT2.
The above cutting apparatus can be used to produce an optical disk as follows:
First, as shown in FIG. 3A, a resist disk RD′ comprising a glass substrate G formed with a resist film R is prepared.
The glass substrate G, for example, has a diameter of 200 mm and a thickness of several mm and is precision polished on its surface.
Further, the resist film R is formed by spin-coating to a thickness of 100 nm using for example a photosensitive positive type photoresist sensitive to ultraviolet rays. The solvent remaining in the resist film R is removed by baking at several tens of degrees Celsius.
Next, as shown in FIG. 3B, the cutting apparatus shown in FIG. 2A and FIG. 2B is used to expose the resist film R by a pattern for exposing regions for forming the recessed regions of the disk substrate and to develop this by an alkali developing solution etc. to form a pattern of the resist film R1 opened at areas for forming the recessed regions in the disk substrate.
Next, as shown in FIG. 4A, for example, the surface is nickel plated to form a stamper 111 on the resist film R1 on the glass substrate G. The surface of the stamper 111 is formed with projecting regions 111a by the transfer of the inverse pattern of relief shapes from the regions R2 for forming recessed regions in the disk substrate opened in the resist film R1. On the other hand, it is formed with recessed regions 111b by the transfer of the inverse pattern of relief shapes in the regions of the resist film R1.
Next, as shown in FIG. 4B, the stamper 111 is removed from the resist film R1 on the glass substrate G.
Next, as shown in FIG. 5A, the above obtained stamper 111 is fixed in a cavity formed by upper and lower molds MD1 and MD2 to form a mold assembly for injection molding. At this time, the stamper 111 is arranged so that its relief shapes forming surface faces the inside of the cavity.
By injecting for example polycarbonate or another resin 112′ in the molten state into the cavity of the injection molding mold assembly from a charging port PT of the mold assembly, as shown in FIG. 5B, a disk substrate 112 is formed over the relief pattern of the stamper 111.
Here, the surface of the disk substrate 112 is formed with projecting regions 112a by the transfer of the inverse pattern of relief shapes from the recessed regions 111b in the surface of the stamper 111, while is formed with recessed regions 112b by the transfer of an inverse pattern of relief shapes from the projecting regions 111a of the stamper 111.
The molded article is removed from the injection molding mold assembly to obtain a disk substrate 112 formed with projecting regions 112a and recessed regions 112b on its surface as shown in FIG. 6A.
Next, as shown in FIG. 6B, the surface of the disk substrate 112 is swept by air, nitrogen gas, or another gas to remove dust, then is successively formed with a dielectric film, a recording film, another dielectric film, and a reflection film in that order by, for example, sputtering, to form the optical recording multilayer film 113.
Next, as shown in FIG. 6C, a protective film 114 is formed above the optical recording multilayer film 113.
Due to the above, an optical disk having the configuration shown in FIG. 1 can be produced.
As the above stamper, it is also possible to use a stamper replicated by transfer of relief shapes from the stamper formed on the resist disk by electroplating as a master, that is, a stamper obtained by forming a mother stamper by electroplating the master and further electroplating the mother stamper.
By making the exposure laser light scan and expose the resist in a spiral while being turned on and off in accordance with the recorded data in the exposure process so as to form pits having lengths in accordance with the recorded data as the relief shapes of the disk substrate 112 and forming an aluminum film or other reflection film as the optical recording film, it is also possible to produce a read only (ROM) type optical disk.
If exposing a resist disk by a predetermined pattern by the cutting apparatus shown in FIGS. 2A and 2B, the mechanism as a whole is placed on an air table so as not to be affected by outside vibration of the place of installation.
In this case, the minimum pattern size able to be formed by the cutting, that is, the resolution P, generally depends on the laser wavelength λ, numerical aperture NA of the object lens, properties of the resist film, etc. and is expressed by the formula (1) from a process factor K normally a value from 0.5 to 0.8:P=K(λ/NA)  (1)
For example, when entering λ=351 nm, NA=0.9, and K=0.5, P=0.2 μm stands, and grooves of a track pitch of 0.4 μm or a pattern of the resist film having a line/space ratio (L/S, widths of portions left as pattern and portions removed) of 0.2 μm/0.2 μm is obtained.
Along with the rapid advances made in information communication and image processing technology in recent years, optical disk recording capacities several times greater than the present will probably be demanded in the near future. This means a recording capacity of, for example, more than 20 GB will be required. To achieve this by the same signal processing as that at present on one surface of a disk having a diameter of 12 cm, a groove pattern having a track pitch of 0.4 μm or less has to be formed on a rewritable optical disk.
Further, in a magneto-optical disk, phase change disk, or other rewritable optical disk, a groove pattern of “deep grooves”, that is, grooves as thin and deep as possible, is preferable for improving the cross erase characteristic at the time of data write operations.
To form such fine, deep grooves, shortening of the laser wavelength λ and an increase of the numerical aperture NA are required from the above formula (1). However, the present value 0.9 of the numerical aperture NA of the object lens is the limit from the viewpoint of the precision in design and fabrication of lenses.
Turning now to the specific problems to be solved by the invention, the stamper used in the above method of production ends up with rough side surfaces of the resist film or surfaces of edges at pits or guide grooves at unexposed portions due to uneven rotation or uneven development of the resist disk at the time of exposure in the method of production. Thus, the surface smoothness is low. This appears as noise in a low frequency range and causes deterioration of the signal-to-noise (S/N) ratio. For example, even if the surface roughness Ra of the original glass substrate is about 0.3 nm, the surface roughness Ra of the nickel stamper obtained therefrom ends up becoming about 0.5 nm reflecting the surface roughness of the resist film.
As methods of smoothing the surface roughness and obtaining low signal noise, there are known the method of ashing by oxygen etc. after exposure and development as described in Japanese Examined Patent Publication (Kokoku) No. 7-29386 and the method of treatment by ultraviolet rays as described in Patent Gazette No. 2506983, however the effects thereof are not sufficient.
Further, the stamper used in the above method of production has to be produced to a thickness of at least 0.25 mm by high speed electroplating, so uneven thickness of the inner and outer circumferences occurs particularly easily. Even when uneven thickness of the inner and outer circumferences of the plating film is suppressed by providing a shielding plate in the plating tank, there is uneven thickness of about 3 μm. In addition, since innumerable granular projections inevitably form on the plating film surface in the above electroplating, back polishing is essential, but scratches caused by the abrasives in the polishing agent end up appearing as macro relief (waviness) on the disk substrate formed due to the filling pressure at the time of injection molding. The residual discrepancy of the focus error caused by the above has become a disadvantage.
Optical disks are being increased in recording capacity by increasing the numerical aperture NA of the object lenses and reducing the track pitch. Along with the higher numerical aperture NA, the depth of focus of the disk and the lens becomes shallower, uneven thickness and slight roughness of the surface of the disk substrate and roughness of the relief signal lead to an increase of focus error and cause signal noise, entering in the spot diameter of the reproduction light. The above disadvantages become particularly serious in a large capacity disk supporting a larger numerical aperture NA.
Further, the stamper used in the above method of production is processed at its inner and outer circumferences by a stamper press, lathe, etc. The circularity after the processing is about 1.5 μm at best at the inner circumference. The disk substrate obtained by inserting the stamper into the mold assembly and injection molding ends up with an offset of the disk center hole and signal portion of as much as about 30 to 70 μm with the addition of the clearance of the mold assembly etc. When the offset of the disk center hole and signal portion becomes large in this way, the crosstalk characteristic ends up deteriorating.